(1) Field of the Invention
The present invention generally relates to semiconductor integrated circuits, and more particularly to a bias circuit for applying bias signals to a differential amplifier, which functions to, for example, amplify an oscillation signal.
(2) Description of the Prior Art
Recently, there has been considerable activity in the development of compact radio transmitter/receiver integrated circuits suitable for mobile communication devices, such as automobile telephones and portable telephones. The radio transmitter/receivers have an oscillation circuit for oscillating a predetermined frequency. In the past, such an oscillation circuit including an oscillation transistor was externally connected to an IC chip in which other circuit portions of the oscillation circuit were formed. However, most recently, an IC chip having a built-in oscillation transistor has been available.
FIG. 1 shows an oscillation transistor Q1 and a differential amplifier which functions as a buffer amplifier. The oscillation transistor Q1 and the buffer amplifier are formed in an IC chip. External structural parts are connected to the IC chip via broken lines, as shown in FIG. 1. A Colpitts type oscillation circuit is made up of the oscillatior transistor Q1, a crystal resonator CR, a capacitor C, a coil CL and a resistor RX. The buffer amplifier is composed of a differential amplifier circuit, a reference bias circuit, an input bias circuit and a current mirror bias circuit.
The differential amplifier circuit is composed of emitter-coupled transistors Q3 and Q4, resistors R3, R4 and R7, and a transistor Q7. The current mirror bias circuit is composed of transistors Q9 and Q10, and resistors R9 and R10. A constant-current circuit is composed of transistors Q7, Q9 and Q10, and resistors R7, R9 and R10. The reference bias circuit is composed of a transistor Q5, resistors R5 and R8, and a transistor Q8. The emitter of the transistor Q5 is connected to the emitter of the transistor Q4, and the collector of the transistor Q5 is connected to a Vcc power supply line. The resistor R5 is connected between the base and collector of the transistor Q5. A constant-current circuit is composed of the transistor Q8, the resistor R8, and the aforementioned current mirror bias circuit. The input bias circuit is composed of a transistor Q2, resistors R2 and R6 and a transistor Q6. The emitter of the transistor Q2 is connected to the base of the transistor Q3, and the collector of the transistor Q2 is connected to the Vcc line. The base of the transistor Q2 is connected to the base of the oscillation transistor Q1 via a resistor R1. The resistor R2 is connected between the base and collector of the transistor Q2. The resistor R6 and the transistor Q6 form, together with the aforementioned current mirror bias circuit, a constant current circuit. The transistors Q2 and Q5 have identical characteristics, and the transistors Q3 and Q4 have identical characteristics. Further, the transistors Q6 -Q9 have identical characteristics.
The external resistor Rx is adjusted to pass a desired amount of current through the oscillation transistor Q1, so that the oscillation circuit oscillates at a desired frequency. The oscillation signal passes through the input buffer circuit, and is applied to the base of the transistor Q3. Complementary amplified oscillation outputs f1 and /f1 are obtained at the collectors of the transistors Q3 and Q4 where "/" means an opposite phase.
However, the conventional circuit shown in FIG. 1 has the following disadvantage. The adjustment of the external resistor Rx to obtain the desired oscillation frequency varies the base current of the oscillation transistor Q1 passing through the resistors R2 and R1. In response to a variation in the base current, the base potential of the transistor Q2 of the input bias circuit varies and thus the base potential of the transistor Q3 varies. Meanwhile, the base potential of the transistor Q4 of the reference bias circuit is fixed. Thus, the amplified oscillation outputs f1 and /f1 have an offset output based on a variation in the base potential of the transistor Q3. The oscillation outputs f1 and /f1 are used as, for example, local input signals of a mixer circuit located in the next stage of the buffer amplifier circuit. The mixer circuit mixes an intermediate frequency signal with the local input signals. In this case, the offset output degrades the frequency conversion characteristics of the mixer circuit.
In order to eliminate the above-mentioned disadvantage, it may be possible to provide a capacitor between the base terminal of the transistor Q2 and a node where the resistors R1 and R2 are connected in series. In this case, the output of the oscillation circuit is capacitively coupled to the input buffer circuit. However, use of such a capacitor requires a large area on the chip, so that the chip size and production costs increase.